US7433111B2 - Electrooptic modulation element - Google Patents

Electrooptic modulation element Download PDF

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US7433111B2
US7433111B2 US10/523,122 US52312205A US7433111B2 US 7433111 B2 US7433111 B2 US 7433111B2 US 52312205 A US52312205 A US 52312205A US 7433111 B2 US7433111 B2 US 7433111B2
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electro
electrodes
optic
optic crystal
modulation device
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US20060051019A1 (en
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Aiichirou Sasaki
Mitsuru Shinagawa
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Nippon Telegraph and Telephone Corp
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Nippon Telegraph and Telephone Corp
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    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/03—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on ceramics or electro-optical crystals, e.g. exhibiting Pockels effect or Kerr effect
    • G02F1/0305—Constructional arrangements
    • G02F1/0316—Electrodes
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R29/00—Arrangements for measuring or indicating electric quantities not covered by groups G01R19/00 - G01R27/00
    • G01R29/08—Measuring electromagnetic field characteristics
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R29/00—Arrangements for measuring or indicating electric quantities not covered by groups G01R19/00 - G01R27/00
    • G01R29/12—Measuring electrostatic fields or voltage-potential
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/03—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on ceramics or electro-optical crystals, e.g. exhibiting Pockels effect or Kerr effect
    • G02F1/035—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on ceramics or electro-optical crystals, e.g. exhibiting Pockels effect or Kerr effect in an optical waveguide structure
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F2202/00—Materials and properties
    • G02F2202/32—Photonic crystals

Definitions

  • the present invention relates to an electro-optic modulation device that includes electro-optic (EO) crystal having an variable birefringence index according to a coupled electric field, and one pair of electrodes disposed so as to have the electro-optic crystal interposed therebetween to couple the electric field to the electro-optic crystal, and that changes polarization of light incident between the one pair of electrodes according to a change of the birefringence index depending upon a strength of electric field coupled via the one pair of electrodes.
  • the present invention relates to an electro-optic modulation device improved in modulation efficiency and sensitivity and flattened in frequency characteristics without hampering the strength and increasing the size.
  • An electro-optic modulation device using electro-optic crystal is used as an electro-optic modulator which modulates the phase of light passed through the crystal according to the magnitude of the electric field generated between electrodes, or as an electric-field sensor for conversely detecting a phase change of light passed through the crystal and thereby detecting the electric field between the electrodes or an electric signal.
  • an optical beam is incident on electro-optic crystal with AC electric field applied thereto and light emitted from the electro-optic crystal is separated into S-polarized light and P-polarized light by a polarizing-beam splitter (hereafter referred to as PBS).
  • PBS polarizing-beam splitter
  • the polarized lights are detected respectively and independently by two photodetectors (hereafter referred to as PD), and a difference between intensities of the S-polarized light and the P-polarized light is detected by the PD and a differential amplifier.
  • FIG. 1 is a diagram showing operation of a conventional electric-field sensor.
  • each polarized component has already been converted to intensity modulated light.
  • the intensity modulated S-polarized component and P-polarized light change in phases opposite to each other. Accordingly, by receiving light in PDs 117 and 119 and conducting differential signal detection in a differential amplifier 121 , therefore, it becomes possible to obtain an output signal 122 having a higher signal-to-noise ratio (see, for example, Japanese Patent Application Laid-Open Nos. 2003-98205, 2003-98204 and 2000-171488).
  • the electro-optic modulation device using electro-optic crystal begins to be applied to communication between wearable computers using a living body as a signal path.
  • a living body as a signal path.
  • communication that does not depend upon the positional relation between the ground of the wearable computer and the earth ground to the utmost, that is communication with a wearable computer that is in an arbitrary position on the living body, can be certainly implemented.
  • FIGS. 2A to 2C are diagrams to explain a process for fabricating an electro-optic modulation device by using electro-optic crystal.
  • An electro-optic modulation device including electro-optic crystal and a pair of electrodes is formed by working thinly electro-optic crystal 107 a of a raw material as shown in FIG. 2A to form a thin electro-optic crystal 107 as shown in FIG. 2B , and forming a pair of electrodes 111 and 113 on a pair of opposite side faces of the electro-optic crystal 107 worked to become thin.
  • an electro-optic crystal 101 a thus worked to become thin has a thickness d of approximately 0.1 mm.
  • ⁇ ⁇ ( V/d ) ⁇ L
  • ⁇ is a constant depending upon the kind of the electro-optic crystal and the structure of the device
  • V is a voltage applied to the electrodes
  • d is a distance between the electrodes
  • L is a length of the electro-optic modulation device.
  • the sensitivity of the electro-optic modulation device serving as an electric-field sensor can be improved by lengthening the length of the electro-optic crystal in a light passage direction as described above. If the electro-optic modulation device is provided with a specific structure in order to increase the intensity by making the electro-optic crystal thin, then a phenomenon that light does not emit from the end face of the electro-optic crystal and light leaks in a side face direction because of light diffraction as the length is made longer is caused, resulting in a lowered modulation efficiency or a lowered sensitivity.
  • An object of the present invention is to provide an electro-optic modulation device capable of improving the modulation efficiency and sensitivity.
  • an object of the present invention is to provide an electro-optic modulation device capable of improving the modulation efficiency and sensitivity without hampering the strength of the device and causing a leak of light due to diffraction even when the gap between the pair of electrodes is made narrow.
  • an object of the invention is to provide an electro-optic modulation device which has flatness in the frequency characteristic.
  • a spirit of invention according to a first aspect is an electro-optic modulation device that includes electro-optic crystal having a birefringence index changed by a coupled electric field, and one pair of electrodes disposed so as to have the electro-optic crystal interposed therebetween to couple the electric field to the electro-optic crystal, and that changes polarization of light incident between the one pair of electrodes according to a change of the birefringence index depending upon a strength of electric field coupled via the one pair of electrodes, wherein the electro-optic crystal includes grooves parallel to a direction of the incident light respectively on one pair of side faces parallel to the direction, and consequently a thin crystal portion sandwiched between the grooves serves as a portion for coupling the electric field, and the one pair of electrodes are formed so as to fill the grooves, respectively.
  • the grooves are formed on the one pair of side faces so as to range from one to the other of end faces through which light is incident or emitted, in the invention according to the first aspect.
  • the grooves are formed in only a central portion except end portions between the end faces through which light is incident or emitted, in the one pair of side faces, in the invention according to the first aspect.
  • the grooves are formed on the one pair of side faces so as to range from one to the other of end faces through which light is incident or emitted, in the invention according to the fourth aspect.
  • the grooves are formed in only a central portion except end portions between the end faces through which light is incident or emitted, in the one pair of side faces, in the invention according to the fourth aspect.
  • remaining portions of the grooves except the one pair of electrode portions are filled with insulators, and a whole of portions except the end faces through which light is incident or emitted is covered by further insulators, in the invention according to the fourth to sixth aspects.
  • the insulators are wax, in the invention according to the fourth to seventh aspects.
  • a spirit of invention according to a ninth aspect is an electro-optic modulation device that includes electro-optic crystal having a birefringence index changed by a coupled electric field, and one pair of electrodes disposed so as to have the electro-optic crystal interposed therebetween to couple the electric field to the electro-optic crystal, and that changes polarization of light incident between the one pair of electrodes according to a change of the birefringence index depending upon a strength of electric field coupled via the one pair of electrodes, the electro-optic modulation device including a base portion, and a ridge-shaped ridge portion projected on one side face of the base portion and extended in a direction of the incident light, at least a part of the ridge portion including the electro-optic crystal, the ridge portion having a width equivalent to a predetermined value or less, wherein the one pair of electrodes are formed on one pair of side faces opposed in a width direction of the ridge portion.
  • the ridge portion is formed nearly in the center on the one side face of the base portion when seen from the direction of the light incidence, in the invention according to the ninth aspect.
  • the ridge portion is formed on an end on the one side face of the base portion when seen from the direction of the light incidence, in the invention according to the ninth aspect.
  • the electro-optic modulation device further includes an insulator which covers the whole, in the invention according to the ninth aspect.
  • the electro-optic modulation device further includes an insulator which covers the ridge portion, in the invention according to the ninth aspect.
  • the electro-optic modulation device further includes an insulator which covers a top surface of the ridge portion and side faces of the one pair of electrodes forming faces continuous to the top surface, in the invention according to the ninth aspect.
  • the insulator includes wax, in the invention according to the twelfth to fourteenth aspects.
  • the ridge portion includes the electro-optic crystal, and the base portion includes the low refractive index medium, in the invention according to the sixteenth aspect.
  • the ridge portion and an upper part of the base portion include the electro-optic crystal, and a remaining lower part of the base portion includes the low refractive index medium, in the invention according to the sixteenth aspect.
  • the low refractive index medium is electro-optic crystal which includes chemical elements of the same kinds as those of the electro-optic crystal, but which is lower in refractive index on the basis of a difference in composition ratio, in the invention according to the seventeenth to nineteenth aspects.
  • the ridge portion and an upper part of the base portion include the electro-optic crystal, a lower part of the electro-optic crystal of the base portion includes an adhesive agent, and a remaining lower part of the base portion includes a substrate, in the invention according to the sixteenth aspect.
  • the base portion includes a substrate, a lower part of the ridge portion includes an adhesive agent, and a remaining upper part of the ridge portion includes the electro-optic crystal, in the invention according to the sixteenth aspect.
  • the low refractive index medium includes gas or a vacuum state in a cavity provided in an upper part of the base portion, in the invention according to the sixteenth aspect.
  • the ridge portion includes the electro-optic crystal, and the base portion includes photonic crystal having a periodic structure, in the invention according to the ninth aspect.
  • a spirit of invention according to a twenty-sixth aspect is an electro-optic modulation device that includes electro-optic crystal having a birefringence index changed by a coupled electric field, and one pair of electrodes disposed so as to have the electro-optic crystal interposed therebetween to couple the electric field to the electro-optic crystal, and that changes polarization of light incident between the one pair of electrodes according to a change of the birefringence index depending upon a strength of electric field coupled via the one pair of electrodes, the electro-optic modulation device further including an insulator applied so as to relatively fix the electro-optic crystal and the one pair of electrodes, except end faces through which light is incident or emitted.
  • the insulator includes a matter that has viscosity and a property of becoming hard with the lapse of time, in the invention according to the twenty-sixth aspect.
  • FIG. 1 is a diagram explaining operation of a conventional electric-field sensor.
  • FIGS. 2A to 2C are diagrams for explain a process for fabricating an electro-optic modulation device by using electro-optic crystal.
  • FIGS. 3A and 3B are respectively a partial perspective oblique view and a sectional view showing an electro-optic modulation device according to an embodiment of the present invention.
  • FIGS. 4A to 4D are diagrams showing a manufacture process of an electro-optic modulation device having a configuration shown in FIG. 3A .
  • FIGS. 5A and 5B are respectively a partial perspective oblique view and a sectional view showing an electro-optic modulation device according to another embodiment of the present invention.
  • FIGS. 6A to 6D are diagrams showing a manufacture process of an electro-optic modulation device having a configuration shown in FIG. 5A .
  • FIGS. 7A and 7B are respectively a partial perspective oblique view and a sectional view showing an electro-optic modulation device according to another embodiment of the present invention.
  • FIGS. 8A to 8D are diagrams showing a manufacture process of an electro-optic modulation device having a configuration shown in FIG. 7A .
  • FIGS. 9A to 9C are a longitudinal sectional view of a structure of an electro-optic modulation device according to the embodiment shown in FIGS. 7A and 7B seen from a direction perpendicular to FIG. 7B , and longitudinal sectional views showing other structures of corner portions.
  • FIGS. 10A to 10D are sectional views showing a manufacture process of an electro-optic modulation device according to another embodiment of the present invention.
  • FIG. 11 is a diagram showing an electro-optic modulation device shown in FIG. 10D with electrodes formed of unnecessary metal remained on electro-optic crystal being removed.
  • FIGS. 12A to 12D are sectional views showing a manufacture process of an electro-optic modulation device according to another embodiment of the present invention.
  • FIG. 13 is a diagram showing a plane of light incidence of an electro-optic modulation device according to an embodiment of ridge type.
  • FIGS. 14A and 14B are diagrams showing a ridge electro-optic modulation device using photonic crystal.
  • FIG. 15 is a diagram showing a plane of light incidence of an electro-optic modulation device according to another embodiment of ridge type.
  • FIG. 16 is a diagram showing a plane of light incidence of an electro-optic modulation device according to another embodiment of ridge type.
  • FIGS. 17A and 17B are diagrams showing a plane of light incidence of an electro-optic modulation device according to another embodiment of ridge type.
  • FIGS. 18A to 18E are diagrams showing how wax is applied to electro-optic crystal placed longitudinally on a pedestal.
  • FIG. 19 is a diagram showing differences in output characteristics of an electric-field sensor among the case where wax is not applied to electro-optic crystal, the case where wax is applied to a top surface of the electro-optic crystal, and the case where wax is applied from the top surface of the electro-optic crystal to both electrodes, and further to a pedestal.
  • FIGS. 20A to 20E are diagrams showing how wax is applied to electro-optic crystal placed laterally on a pedestal.
  • FIGS. 21A and 21B are diagrams showing how wax is applied to an electro-optic modulation device of the so-called H-type.
  • FIGS. 22A to 22C are diagrams showing how wax is applied to an electro-optic modulation device of the so-called ridge type.
  • FIGS. 3A and 3B are respectively a partial perspective oblique view and a sectional view showing an electro-optic modulation device according to an embodiment of the present invention.
  • the electro-optic modulation device includes electro-optic crystal 1 having a pair of grooves dug and formed in a side face 1 a and a side face 1 b opposite to the side face 1 a in their longitudinal direction so as to extend from an end face 1 c to an end face 1 d , and a pair of electrodes 5 a and 5 b formed of metal embedded in the pair of grooves.
  • each of the pair of grooves has a rectangular sectional shape, and each of the pair of electrodes 5 a and 5 b buries its groove nearly completely.
  • FIG. 3B each of the pair of grooves has a rectangular sectional shape, and each of the pair of electrodes 5 a and 5 b buries its groove nearly completely.
  • the electro-optic modulation device according to the present embodiment has a rectangular sectional shape, when the electro-optic crystal 1 and the pair of electrodes 5 a and 5 b are seen collectively. Furthermore, the electro-optic modulation device according to the present embodiment is sometimes called “H-type” on the basis of the sectional shape of the electro-optic crystal 1 .
  • the pair of grooves are formed by, for example, digging both the side faces 1 a and 1 b by means of cutting or polishing so as to make bottoms approach each other in order to make a distance d between the pair of electrodes 5 a and 5 b equal to a predetermined distance or less.
  • the distance d between the electrodes 5 a and 5 b is 0.1 mm or less
  • the length L is approximately 2 cm
  • dimensions t and x of the section respectively in the longitudinal and lateral directions are approximately 1 cm or less.
  • the electro-optic modulation device having such a configuration is formed so as to have an extremely small distance d between the pair of electrodes 5 a and 5 b , the electrodes 5 a and 5 b are formed so as to nearly completely embed the grooves formed in the electro-optic crystal 1 as a whole and the thin crystal portion between the electrodes 5 a and 5 b is formed so as to be generally covered by the electrodes 5 a and 5 b and the electro-optic crystal 1 . Therefore, the electro-optic crystal 1 is not easily broken from the thin portion between the electrodes 5 a and 5 b .
  • the thin structure between the electrodes 5 a and 5 b is also formed by cutting or polishing the electro-optic crystal 1 of the raw material from both side faces 1 a and 1 b . Therefore, it is not difficult to conduct working so as to make the portion between the electrodes 5 a and 5 b extremely thin, for example, 0.1 mm or less.
  • a spot beam 123 is incident from an end face of the electro-optic crystal 1 between the electrodes 5 a and 5 b .
  • antireflection coating on the plane of incidence as well, it can be conducted extremely easily and certainly by applying the antireflection coating to not only the end face of the thin crystal portion between the electrodes 5 a and 5 b , but also on an end face 1 c of the whole rectangular electro-optic modulation device including the end face of the electro-optic crystal 1 except the thin crystal portion and end faces of the electrodes 5 a and 5 b.
  • the electrodes 5 a and 5 b , and the thin crystal portion between the electrodes 5 a and 5 b are fixed by the whole electro-optic crystal. These results in an effect that distortion of the thin crystal portion is suppressed and the frequency characteristics become flat.
  • the direction of an electric-field vector generated by the electrodes 5 a and 5 b is perpendicular to the opposed planes of the electrodes 5 a and 5 b.
  • FIGS. 4A to 4D A manufacture method of the electro-optic modulation device having the configuration shown in FIGS. 3A and 3B will now be described with reference to FIGS. 4A to 4D .
  • this electro-optic modulation device for example, rectangular electro-optic crystal 1 of the raw material shown in FIG. 2A is first dug from its both side faces 1 a and 1 b by cutting or polishing to form two grooves 3 a and 3 b as shown in FIG. 4A .
  • metal such as silver paste is thinly applied to the grooves 3 a and 3 b as represented by characters 5 aa and 5 ba in FIG. 4B to form thin electrodes 5 aa and 5 ba .
  • lead wires 53 for applying a voltage are adhered to the electrodes 5 aa and 5 ba.
  • the electro-optic modulation device is formed.
  • the electrodes 5 aa and 5 ba have thin structures.
  • silver paste is further applied onto the electrodes 5 aa and 5 ba formed of silver paste to fill the gaps.
  • an electro-optic modulation device including the electrodes 5 a and 5 b that are equal in thickness to those shown in FIG. 3A is completed.
  • FIGS. 5A and 5B are respectively a partial perspective oblique view and a sectional view showing an electro-optic modulation device according to another embodiment of the present invention.
  • Electrodes 7 a and 7 b having a thickness less than a predetermined thickness that is less than the depth of grooves are formed on bottoms in the grooves 3 a and 3 b , instead of the electrodes 5 a and 5 b generally formed in the grooves 3 a and 3 b in the electro-optic modulation device according to the embodiment shown in FIGS. 3A and 3B , and insulators 9 a and 9 b are formed so as to fill the grooves left above the thin electrodes 7 a and 7 b and thereby form the whole electro-optic modulation device as one body.
  • the electrodes 7 a and 7 b are formed so as to nearly completely fill the grooves 3 a and 3 b formed in the electro-optic crystal 1 as a whole in conjunction with the insulators 9 a and 9 b and generally cover the thin crystal portion between the electrodes 7 a and 7 b by the electrodes 7 a and 7 b , the insulators 9 a and 9 b and the electro-optic crystal 1 . Therefore, the electro-optic crystal 1 is not broken easily from the thin portion of the electrodes 7 a and 7 b .
  • the thin crystal structure between the electrodes 7 a and 7 b respectively in the grooves 3 a and 3 b is also formed by digging the electro-optic crystal 1 of the raw material from both side faces 1 a and 1 b by means of cutting or polishing, it is not difficult to conduct working so as to make the portion between the electrodes 7 a and 7 b extremely thin, for example, 0.1 mm or less.
  • a spot beam 123 is incident from an end face of the electro-optic crystal 1 between the electrodes 7 a and 7 b .
  • antireflection coating on the plane of incidence as well, it can be conducted extremely easily and certainly by applying the antireflection coating to not only the end face of the thin crystal portion between the electrodes 7 a and 7 b , but also on an end face 1 c of the whole rectangular electro-optic modulation device including the end face of the electro-optic crystal 1 except the thin crystal portion and end faces of the electrodes 7 a and 7 b , and end faces of the insulators 9 a and 9 b.
  • the electrodes 7 a and 7 b , and the thin crystal portion between the electrodes 7 a and 7 b are fixed by the whole electro-optic crystal and the insulators 9 a and 9 b . These results in an effect that distortion of the thin crystal portion is suppressed and the frequency characteristics become flat.
  • the direction of an electric-field vector generated by the electrodes 7 a and 7 b is perpendicular to the opposed planes of the electrodes 7 a and 7 b.
  • FIGS. 6A to 6D A manufacture method of the electro-optic modulation device having the configuration shown in FIGS. 5A and 5B will now be described with reference to FIGS. 6A to 6D .
  • this electro-optic modulation device for example, rectangular electro-optic crystal 1 of the raw material shown in FIG. 2A is first dug from its both side faces 1 a and 1 b by cutting or polishing to form two grooves 3 a and 3 b as shown in FIG. 6A .
  • a conductive material such as silver paste is thinly applied to the grooves 3 a and 3 b to form electrodes 7 a and 7 b as shown in FIG. 6B .
  • a conductive material such as silver paste is thinly applied to the grooves 3 a and 3 b to form electrodes 7 a and 7 b as shown in FIG. 6B .
  • lead wires 53 for applying a voltage are adhered to the electrodes 7 a and 7 b .
  • the grooves above the thin electrodes 7 a and 7 b having the lead wires 53 adhered thereto are filled with the insulators 9 a and 9 b , leaving no space.
  • an adhesive agent is suitable as the insulators 9 a and 9 b.
  • FIGS. 7A and 7B are respectively a partial perspective oblique view and a sectional view showing an electro-optic modulation device according to another embodiment of the present invention.
  • the electro-optic modulation device according to the embodiment shown in FIGS. 7A and 7C is formed so as to have such a sandwich structure only in a central portion between the end face 1 c and the end face 1 d except end portions.
  • concave portions 4 a and 4 b surrounded by the electro-optic crystal in their periphery are formed, and the sandwich structure is formed in the concave portions 4 a and 4 b .
  • Other structures and operations are the same as those in the embodiment shown in FIGS. 5A and 5B .
  • the electrodes 7 aa and 7 ba are formed so as to nearly completely embed the concave portions 4 a and 4 b formed in the electro-optic crystal 1 as a whole in conjunction with insulators 9 aa and 9 ba and the thin crystal portion between the electrodes 7 aa and 7 ba is formed so as to be generally covered by the electrodes 7 aa and 7 ba , the insulators 9 aa and 9 ba , and the electro-optic crystal 1 . Therefore, the electro-optic crystal 1 is not easily broken from the thin portion between the electrodes 7 aa and 7 ba .
  • the thin structure between the electrodes 7 aa and 7 ba in the concave portions 4 a and 4 b is also formed by digging the electro-optic crystal 1 of the raw material from both side faces 1 a and 1 b by means of cutting or polishing. Therefore, it is not difficult to conduct working so as to make the distance d between the electrodes 7 aa and 7 ba extremely short, for example, 0.1 mm or less.
  • a spot beam 123 is incident on an end face of the electro-optic crystal 1 between the electrodes 7 aa and 7 ba from the end face of the electro-optic crystal 1 generally covering its outside.
  • antireflection coating on the plane of incidence as well, it can be conducted extremely easily and certainly because the antireflection coating is conducted on the whole end face of the electro-optic crystal 1 .
  • the electrodes 7 aa and 7 ba , and the thin crystal portion between the electrodes 7 aa and 7 ba are fixed by the whole electro-optic crystal and the insulators 9 aa and 9 ba . These results in an effect that distortion of the thin crystal portion is suppressed and the frequency characteristics become flat.
  • the direction of an electric-field vector generated by the electrodes 7 aa and 7 ba is perpendicular to the opposed planes of the electrodes 7 aa and 5 ba.
  • a manufacture method of the electro-optic modulation device having the configuration shown in FIGS. 7A and 7B will now be described with reference to FIGS. 8A to 8D .
  • rectangular electro-optic crystal of the raw material is first dug from its both side faces 1 a and 1 b by cutting or polishing to form two concave portions 4 a and 4 b each taking the shape of a groove, as shown in FIG. 8A .
  • a conductive material such as silver paste is thinly applied to the concave portions 4 a and 4 b to form electrodes 7 aa and 7 ba as shown in FIG. 8B .
  • a conductive material such as silver paste is thinly applied to the concave portions 4 a and 4 b to form electrodes 7 aa and 7 ba as shown in FIG. 8B .
  • lead wires 53 for applying a voltage are adhered to the electrodes 7 aa and 7 ba.
  • the concave portions above the thin electrodes 7 aa and 7 ba having the lead wires 53 adhered thereto are filled by the insulators 9 aa and 9 ba , leaving no space.
  • an adhesive agent is suitable as the insulators 9 aa and 9 ba .
  • the electro-optic modulation device shown in FIGS. 7A and 7B can be manufactured by using the same manufacture method as that of the electro-optic modulation device shown in FIGS. 5A and 5B except that the grooves are replaced by concave portions.
  • the concave portions 4 a and 4 b formed in the electro-optic crystal it is not necessary that sides thereof are at right angles, but the sides may be inclined or curved as shown in FIGS. 9B and 9C and as described below.
  • FIG. 9A is a longitudinal sectional view of the structure of the electro-optic modulation device according to the embodiment shown in FIGS. 7A and 7B seen from a direction perpendicular to FIG. 7B .
  • all corner portions 11 of bottoms of the concave portions 4 a and 4 b having the pair of electrodes 7 aa and 7 ba and the insulators 9 aa and 9 ba embedded therein are formed nearly at right angles.
  • FIGS. 9B and 9C are longitudinal sectional views showing other structures of the corner portions.
  • all corner portions on bottoms in the concave portions 4 aa and 4 ba are formed so as to be inclined at an obtuse angle which is larger than a right angle.
  • all corner portions on bottoms in the concave portions 4 aa and 4 ba are formed so as to be curved round without being angular.
  • the concave portions may be filled with only a conductive material to form electrodes.
  • FIGS. 10A to 10D are sectional views showing a manufacture process of an electro-optic modulation device according to another embodiment of the present invention.
  • the electro-optic modulation device finally includes a ridge portion 21 formed on the top surface of the electro-optic crystal 1 so as to project with a predetermined width d or less, for example, with 0.1 mm or less, and a pair of electrodes 25 a and 25 b formed on a pair of side faces opposed to each other in the width direction of the ridge portion 21 , as shown in FIG. 10D .
  • the top surface of the electro-optic crystal 1 of the raw material shown in FIG. 10A is first cut or polished as shown in FIG. 10B to form the ridge portion 21 having a width equal to a predetermined width d or less, for example, 0.1 mm or less.
  • metal 23 is deposited by evaporation or applied to the top surface of the electro-optic crystal 1 having the formed ridge portion 21 .
  • FIG. 10D only the metal 23 deposited on the ridge portion 21 by evaporation is removed by polishing or the like. As a result, the pair of electrodes 25 a and 25 b are formed with metal left on both side faces of the ridge portion 21 .
  • the electrodes 25 a and 25 b are formed on the projected portion of the electro-optic crystal 1 which is large as a whole. Therefore, the electro-optic crystal between the electrodes 25 a and 25 b is not easily broken.
  • the electrodes 25 a and 25 b are formed by conducting working and metal evaporation on the top surface of the electro-optic crystal 1 . Therefore, it is not difficult to conduct working so as to make the distance d between the electrodes 25 a and 25 b located across the ridge portion 21 extremely thin, for example, 0.1 mm or less.
  • a spot beam 123 is incident from an end face of the electro-optic crystal 1 between the electrodes 25 a and 25 b .
  • antireflection coating on the plane of incidence as well, it can be conducted extremely easily and certainly by applying the antireflection coating to not only the end face of the thin crystal portion between the electrodes 25 a and 25 b , but also on an end face of the whole electro-optic modulation device including the end face of the electro-optic crystal 1 formed as one body below the portion.
  • the direction of an electric-field vector generated by the electrodes 25 a and 25 b is perpendicular to the opposed planes of the electrodes 25 a and 25 b.
  • the metal 23 deposited on the ridge portion 21 by evaporation is removed by polishing or the like, and the pair of electrodes 25 a and 25 b are formed of the metal left on both side faces of the ridge portion 21 .
  • the metal 23 remains on the top surface of the electro-optic crystal 1 besides the side faces opposed to each other across the ridge portion 23 , and this portion also acts as electrodes.
  • undesired electric fields generated between electrodes of this portion are extremely few, and a large majority is generated between the opposed electrodes 25 a and 25 b on the ridge portion 21 .
  • metal on the remaining portion is removed as shown in, for example, FIG. 11 in order to remove slight or unnecessary electric fields generated between electrodes formed on the portion of remaining metal, generation of such unnecessary electric fields can be avoided.
  • the metal of that portion is not removed daringly, an advantage that the mechanical strength is conversely increased is obtained.
  • FIGS. 12A to 12D are sectional views showing a manufacture process of an electro-optic modulation device according to another embodiment of the present invention.
  • the electro-optic modulation device finally includes a ridge portion 21 a formed on one end of the top surface of the electro-optic crystal 1 so as to project with a predetermined width d or less, for example, with 0.1 mm or less, and a pair of electrodes 29 a and 29 b formed on a pair of side faces opposed to each other in the width direction of the ridge portion 21 a , as shown in FIG. 12D .
  • the top surface of the electro-optic crystal 1 of the raw material shown in FIG. 12A is first cut or polished as shown in FIG. 12B to form the ridge portion 21 a having a width equal to a predetermined width d or less, for example, 0.1 mm or less.
  • metal 27 is deposited by evaporation or applied to the top surface of the electro-optic crystal 1 having the formed ridge portion 21 and a side face on which the ridge portion 21 a is formed inclusive of the ridge portion 21 a .
  • FIG. 12D only the metal 27 deposited on the ridge portion 21 a by evaporation is removed by polishing or the like. As a result, the pair of electrodes 29 a and 29 b are formed with metal left on both side faces of the ridge portion 21 a.
  • the electrodes 29 a and 29 b are formed on the projected portion of the electro-optic crystal 1 which is large as a whole in the same way as the embodiment shown in FIG. 10D . Therefore, the electro-optic crystal between the electrodes 29 a and 29 b is not easily broken.
  • the electrodes 29 a and 29 b are formed by conducting working and metal evaporation on the top surface of the electro-optic crystal 1 . Therefore, it is not difficult to conduct working so as to make the distance d between the electrodes 29 a and 29 b located across the ridge portion 21 a extremely thin, for example, 0.1 mm or less.
  • a spot beam 123 is incident from an end face of the electro-optic crystal 1 between the electrodes 25 a and 25 b .
  • antireflection coating on the plane of incidence as well, it can be conducted extremely easily and certainly by applying the antireflection coating to not only the end face of the thin crystal portion between the electrodes 29 a and 29 b , but also on an end face of the whole electro-optic modulation device including the end face of the electro-optic crystal 1 formed as one body below the portion.
  • the direction of an electric-field vector generated by the electrodes 29 a and 29 b is perpendicular to the opposed planes of the electrodes 29 a and 29 b.
  • the metal 27 deposited on the ridge portion 21 a by evaporation is removed by polishing or the like, and the pair of electrodes 29 a and 29 b are formed of the metal left on both side faces of the ridge portion 21 a .
  • the metal 27 remains on the top surface of the electro-optic crystal 1 and on side faces besides the side faces opposed to each other across the ridge portion 21 a and this portion also acts as electrodes.
  • undesired electric fields generated between electrodes of this portion are extremely few, and a large majority is generated between the opposed electrodes 29 a and 29 b on the ridge portion 21 a.
  • the light diffraction effect becomes an obstacle.
  • L when L is small, light is emitted from the end face of the electro-optic crystal even if it is it diffracted and consequently there is no light loss.
  • L is made large, diffracted light proceeds in such a direction as to get out of the ridge portion 21 ( 21 a ).
  • the top surface of the ridge portion 21 ( 21 a ) is in contact with air and both side faces are in contact with the electrodes. At these faces, therefore, reflection takes place and consequently light does not leak.
  • the electro-optic crystal 1 which is the same as the ridge portion 21 ( 21 a ) is present under the ridge portion 21 ( 21 a ), however, light leakage from the ridge portion 21 ( 21 a ) occurs. If the length of the electro-optic crystal is lengthened, therefore, a large phase modulation depth and a large electric-field sensitivity corresponding to the length cannot be obtained.
  • FIG. 13 is a diagram showing a plane of light incidence of an electro-optic modulation device according to an embodiment of ridge type.
  • the electro-optic modulation device includes electro-optic crystal 61 changed in birefringence index by electric-field coupling, and a low refractive index medium 62 having a refractive index that is less than the refractive index of the electro-optic crystal 61 . It is desirable that the refractive index of the low refractive index medium 62 is lower than that of the electro-optic crystal 61 by at least approximately 10%. For example, if the refractive index of the electro-optic crystal 61 is 3, the refractive index of the low refractive index medium 62 should be 2.7 or less. In general, the larger the difference in refractive index between the electro-optic crystal 61 and the low refractive index medium 62 becomes, the more desirable.
  • the electro-optic crystal 61 is formed of, for example, GaAs (gallium arsenide), InP (indium phosphide), CdTe (cadmium telluride) or ZnTe (zinc telluride).
  • the electro-optic crystal 61 is sandwiched between the open air above the top surface 61 a and the low refractive index medium 62 .
  • the L-shaped electrodes 65 a and 65 b are provided so as to extend over the side faces 64 a and 64 b and the top surface 63 a of the base portion 63 .
  • the mechanical strength is improved.
  • the possibility of the base portion 63 and the ridge portion 64 being separated from each other or a part of the ridge portion 64 being damaged can be reduced.
  • the electro-optic crystal 61 is equivalent to a core in the optical waveguide, and the low refractive index medium 62 is equivalent to a clad in the optical waveguide.
  • the electro-optic modulation device in the embodiment, light can be trapped in the electro-optic crystal 61 . Even if the length of the electro-optic crystal 61 in the z-direction is lengthened, therefore, it is possible to prevent diffracted light from being leaked. As a result, a large phase modulation depth and a large electric-field sensitivity can be obtained.
  • the base portion 63 is formed of the low refractive index medium 62 and the ridge portion 64 is formed of the electro-optic crystal 61 .
  • the structure can be simplified and consequently the manufacture of the electro-optic modulation device is facilitated.
  • it is facilitated to manufacture the base portion 63 and the ridge portion 64 separately and couple them later.
  • a projection is not formed in the low refractive index medium 62 and the possibility of damage in the base portion 63 and the ridge portion 64 can be made low.
  • a lower part of the ridge portion 64 is formed of the low refractive index medium 62 . If the refractive index of the low refractive index medium 62 is not so small as compared with the refractive index of the electro-optic crystal 61 , oozing out of light into the low refractive index medium 62 becomes comparatively large. In this example, the electric field is coupled to the light that has oozed out, as well. If the low refractive index medium 62 forming the lower part of the ridge portion 64 has an electro-optic effect, therefore, the detection sensitivity can be made high. Furthermore, unlike the left side example and the right side example in which the electrodes 65 a and 65 b do not face the light that has oozed out, the sensitivity is not lowered in this example.
  • an upper part of the base portion 63 is formed of the electro-optic crystal 61 , and consequently the electro-optic crystal 61 becomes large.
  • the projection area from the upward becomes large.
  • the mechanical strength of the electro-optic modulation device can be increased.
  • the ridge portion is formed as thin as possible.
  • the electro-optic crystal 61 forming the ridge portion 64 becomes large as a whole and consequently it becomes easy to handle the ridge portion 64 . For example, therefore, the work of conducting antireflection coating on the end face of the electro-optic crystal 61 is facilitated.
  • photonic crystal having a periodic structure instead of the low refractive index medium 62 according to the embodiment.
  • the photonic crystal is a generic term of materials having a periodic structure of a light wavelength order.
  • the photonic crystal has a property of preventing light from entering a region having a periodic structure.
  • an electro-optic modulation device including a ridge portion 71 a formed of a region having no periodic structure and a base portion 71 b formed of a region having a periodic structure by conducting cutting working on electro-optic crystal including a region having no periodic structure and a region having a periodic structure.
  • An electro-optic modulation device may be constructed by adhering electro-optic crystal 73 and photonic crystal 75 having a periodic structure to each other with an adhesive agent 77 as shown in FIG. 14B and then cutting the electro-optic crystal 73 .
  • FIG. 15 is a diagram showing a plane of light incidence of an electro-optic modulation device according to another embodiment of ridge type.
  • kinds of chemical elements included in the low refractive index medium in the embodiment described earlier are made the same as kinds of chemical elements included in the electro-optic crystal, and refractive indexes are made different from each other according to a difference in composition ratio of the chemical elements.
  • Other configurations and differences among examples, operation and effects are not different from those of the electro-optic modulation device in the above-described embodiment, and consequently description of them will be omitted.
  • kinds of chemical elements included in the low refractive index medium in the embodiment described earlier are made the same as kinds of chemical elements included in the electro-optic crystal.
  • the electro-optic crystal can be formed continuously.
  • integral electro-optic crystal 61 A including a high refractive index layer and a low refractive index layer is obtained.
  • manufacture is fabricated. Furthermore, thickness adjustment of the low refractive index medium and the electro-optic crystal can be conducted easily.
  • a boundary plane between the low refractive index medium and the electro-optic crystal can be made similar to an ideal plane, light leak can be reduced as compared with the case where there are a large number of concavities and convexities on this boundary plane.
  • FIG. 16 is a diagram showing a plane of light incidence of an electro-optic modulation device according to another embodiment of ridge type.
  • the electro-optic modulation device includes electro-optic crystal 61 , and an adhesive agent 62 a serving as a low refractive index medium having a refractive index that is less than the refractive index of the electro-optic crystal 61 .
  • the electro-optic modulation device includes a base portion 63 , a ridge portion 64 formed thinly on a top surface 63 a of the base portion 63 so as to include at least the electro-optic crystal 61 and have a top surface 61 a exposed to the open air, and L-shaped electrodes 65 a and 65 b each of which extends over opposed side faces 64 a and 64 b of the ridge portion 64 and the top surface 63 a of the base portion 63 .
  • the electro-optic crystal 61 is formed to be sandwiched between the open air above the top surface 61 a and the adhesive agent 62 a.
  • the L-shaped electrodes 65 a and 65 b are provided so as to extend over the side faces 64 a and 64 b and the top surface 63 a of the base portion 63 . As compared with the case where the electrodes 65 a and 65 b are provided respectively only on the side faces 64 a and 64 b , therefore, the mechanical strength is improved.
  • an optical waveguide is constructed, and consequently light can be trapped in the electro-optic crystal 61 .
  • By lengthening the length of the electro-optic crystal 61 therefore, it becomes possible to obtain a large phase modulation depth and a large electric-field sensitivity corresponding to the length.
  • the substrate 66 a and the electro-optic crystal 61 can be coupled by using the adhesive agent 62 a.
  • the base portion 63 is formed of the substrate 66 and the adhesive agent 62 a disposed above the substrate 66 , and the ridge portion 64 is formed of the electro-optic crystal 61 .
  • the area of contact with the electrodes 65 a and 65 b becomes wide. Therefore, the electrodes 65 a and 65 b can be fixed firmly. Furthermore, it becomes unnecessary to use another adhesive agent to fix the electrodes 65 a and 65 b.
  • the base portion 63 is formed of the substrate 66 , and a lower part of the ridge portion 64 is formed of the adhesive agent 62 a . Therefore, operation and effects similar to those in the example shown in the center in another embodiment are obtained.
  • the base portion 63 is formed of the substrate 66 , the adhesive agent 62 a disposed above the substrate 66 , and the electro-optic crystal 61 disposed above the adhesive agent 62 a , and the ridge portion 64 is formed of the electro-optic crystal 61 . Therefore, operation and effects similar to those in the example shown in the right side in another embodiment are obtained.
  • the low refractive index medium 62 or the adhesive agent 62 a are provided on the bottom surface of the electro-optic crystal 61 which is one of surfaces that extend along the path of light, and the top surface 61 a is exposed to the open air.
  • the electro-optic crystal 61 may be sandwiched between low refractive index media by providing a low refractive medium on the top surface 61 a as well.
  • FIGS. 17A and 17B are diagrams showing a manufacture method of an electro-optic modulation device according to another embodiment of ridge type.
  • an electro-optic modulation device having a cavity 81 a under a ridge portion 81 b may be constructed by, for example, cutting electro-optic crystal 81 having the cavity 81 a formed by the crystal growth process as shown in FIG. 17A .
  • an electro-optic modulation device having a cavity 89 under a ridge portion may be constructed by adhering electro-optic crystal 83 having the ridge portion formed therein previously and a base portion 85 having a hollow formed in its top portion to each other by means of an adhesive agent 87 .
  • cavities 81 a and 89 As gas having a refractive index lower than the refractive index of the electro-optic crystal, for example, air or gas can be sealed in these cavities 81 a and 89 . It is possible to cause the open air to flow into and out of these cavities 81 a and 89 . These cavities 81 a and 89 can be made vacuous.
  • the electro-optic crystal is distorted mainly in a direction perpendicular to the electrode plane, and consequently flatness in frequency characteristics can not be obtained.
  • the distortion of the electro-optic crystal is reduced by wax or the like.
  • FIGS. 18A to 18E are diagrams showing how wax is applied to electro-optic crystal placed longitudinally on a pedestal 19 .
  • FIG. 18A shows the case where wax 37 is applied so as to heap both electrodes 33 and 35 with the wax 37 from a top surface of the electro-optic crystal 31 and in addition heap the pedestal 19 as well with the wax 37 .
  • the distortion of the electro-optic crystal 31 can be suppressed certainly.
  • FIG. 18B shows the case where wax 37 is applied so as to heap one electrode 33 with the wax 37 from a top surface of the electro-optic crystal 31 and in addition heap the pedestal 19 as well with the wax 37 .
  • the distortion of the electro-optic crystal 31 can be suppressed comparatively certainly.
  • the electrode 35 side may be heaped with the wax 37 .
  • FIG. 18A shows the case where wax 37 is applied so as to heap both electrodes 33 and 35 with the wax 37 from a top surface of the electro-optic crystal 31 and in addition heap the pedestal 19 as well with the wax 37 .
  • the distortion of the electro-optic crystal 31 can be suppressed comparatively certainly.
  • the electrode 35 side may be heaped with the
  • FIG. 18C shows the case where wax 37 is applied so as to heap a pedestal 19 with the wax from both electrodes 33 and 35 .
  • FIG. 18D shows the case where wax 37 is applied so as to heap the top surface of the electro-optic crystal 31 with the wax 37 . If the electrodes 33 and 35 are thus fixed to the electro-optic crystal 31 , distortion of the electro-optic crystal 31 can be suppressed.
  • FIG. 18E shows the case where wax 37 is applied so as to heap the top surface of the electro-optic crystal 31 and top end portions of the both electrodes 33 and 35 with the wax 37 .
  • FIG. 19 is a diagram showing differences in output characteristics of an electric-field sensor among the case where wax is not applied to the electro-optic crystal, the case where the top surface of the electro-optic crystal 31 is heaped with wax 37 as shown in FIG. 18D , and the case where both electrodes 33 and 35 and further the pedestal 19 are heaped with wax 37 from the top surface of the electro-optic crystal 31 .
  • the amplitude voltage (output amplitude voltage) of the output signal 122 is flat.
  • resonance is found near 590 kHz, near 610 kHz and near 720 kHz. If the wax 37 is applied to the top surface of the electro-optic crystal 31 as shown in FIG. 18D , however, resonance can be reduced while maintaining the output amplitude voltage. Furthermore, if the wax 37 is applied to the top surface of the electro-optic crystal 31 and both the electrodes 33 and 35 as shown in FIG. 18A , resonance can be eliminated although the output amplitude voltage becomes low.
  • FIGS. 20A to 20E are diagrams showing how wax is applied to electro-optic crystal placed laterally on the pedestal 19 .
  • FIG. 20A shows the case where the wax 37 is applied so as to heap both side faces of the electro-optic crystal 31 with the wax 37 from an electrode 33 disposed on the electro-optic crystal 31 and further heap the pedestal 19 as well with the wax 37 .
  • the distortion of the electro-optic crystal 31 can be suppressed certainly.
  • FIG. 20B shows the case where wax 37 is applied so as to heap one of the side faces of the electro-optic crystal 31 with the wax 37 from a top surface of the electrode 33 and in addition heap the pedestal 19 as well with the wax 37 . In this case as well, the distortion of the electro-optic crystal 31 can be suppressed comparatively certainly.
  • FIG. 20C shows the case where wax 37 is applied so as to heap the pedestal 19 with the wax from both side faces of the electro-optic crystal 31 .
  • the electro-optic crystal 31 can be fixed to both the electrodes 33 and 35 , and can be further fixed to the pedestal 19 as well. Therefore, distortion of the electro-optic crystal 31 can be suppressed comparatively certainly.
  • FIG. 20D shows the case where wax 37 is applied so as to heap both side faces of the electro-optic crystal 31 with wax 37 from an end portion of the electrode 33 and heap the pedestal 19 with the wax 37 from both side faces of the electro-optic crystal 31 .
  • FIG. 20E shows the case where wax 37 is applied so as to heap both side faces of the electro-optic crystal 31 with the wax 37 . Since the electro-optic crystal 31 is fixed to both electrodes 33 and 35 by the wax 37 , distortion of the electro-optic crystal 31 can be suppressed. By the way, wax is not applied to a beam spot BS or the surface of the beam spot. This aims at preventing an optical beam from being diffracted by wax.
  • FIGS. 21A and 21B are diagrams showing how wax is applied to an electro-optic modulation device of the above-described so-called H-type.
  • the insulators 9 a and 9 b in the electro-optic modulation device of H-type shown in FIGS. 5A and 5B are specifically replaced by wax 10 a and 10 b .
  • electrodes 7 a and 7 b are formed with a central thin crystal portion sandwiched therebetween in grooves 3 a and 3 b of the electro-optic crystal 1 placed on the pedestal 19 in the same way as the embodiment shown in FIGS. 5A and 5B .
  • wax 10 a and 10 b are embedded in remaining groove portions as a concrete example of the insulators 9 a and 9 b.
  • the central thin crystal portion is completely surrounded and fixed by the electrodes 7 a and 7 b , wax 10 a and 10 b , and other electro-optic crystal. Therefore, distortion of the thin crystal portion sandwiched between the electrodes 7 a and 7 b can be suppressed.
  • the aspect shown in FIG. 21 further has an effect of complementing the physical strength of the central thin crystal portion in the same way as the aspect shown in FIGS. 5A and 5B .
  • the whole of the electro-optic modulation device of H-type inclusive of the grooves 3 a and 3 b of the electro-optic crystal 1 is covered by wax 10 , and the electro-optic modulation device is fixed to the pedestal 19 by the covering wax 10 .
  • the applied material is not restricted to the wax 10 , but another insulator may be used.
  • FIGS. 22A to 22C are diagrams showing how wax is applied to an electro-optic modulation device of the so-called ridge type.
  • wax 10 is applied to a top surface of the ridge portion 21 including electrodes of the electro-optic modulation device of ridge type shown in FIG. 10D .
  • the ridge portion 21 and the electrodes 25 a and 25 b can be fixed. Therefore, the distortion of the crystal in the ridge portion 21 can be suppressed.
  • the whole of the ridge portion 21 and the electrodes 25 a and 25 b of the electro-optic modulation device of ridge type shown in FIG. 10D is covered by wax 10 .
  • the ridge portion 21 and the electrodes 25 a and 25 b can be fixed. Therefore, the distortion of the crystal in the ridge portion 21 can be suppressed.
  • the whole of the electro-optic modulation device of ridge type shown in FIG. 10D is covered by wax 10 and fixed to the pedestal 19 . It is a matter of course that in this case well distortion of the crystal in the ridge portion 21 can be suppressed.
  • the ridge portion and so on is covered by wax.
  • the ridge portion and so on may be covered by another insulator.
  • Wax becomes hard by evaporation of moisture with elapse of time.
  • a matter that becomes low in temperature and consequently becomes hard with the lapse of time i.e., a matter preheated so as to have viscosity may be used.
  • An adhesive agent may be used.
  • the electro-optic crystal includes grooves formed respectively on one pair of side faces that are parallel to a direction of light incident between a pair of electrodes, so as to become parallel to the direction, and consequently a thin crystal portion sandwiched between the grooves serves as a portion for coupling the electric field.
  • the grooves are filled with one pair of electrodes, or filled with one pair of electrodes and insulators. Therefore, the electro-optic crystal is not easily broken from the thin crystal portion between the electrodes. In addition, it is not difficult to work the electro-optic crystal between the electrodes so as to make it extremely thin.
  • a ridge portion having a width shorter than a predetermined width projected on one side face of a base portion is formed as electro-optic crystal coupled to electric field. Therefore, a thin crystal portion between the electrodes is not easily broken. In addition, it is not difficult to conduct working so as to make the ridge portion between the electrodes extremely thin, for example, 0.1 mm or less.
  • antireflection coating on the plane of incidence as well, it can be conducted extremely easily and certainly by generally applying the antireflection coating to not only the end face of the thin crystal portion between the electrodes, but also on an end face of the whole electro-optic modulation device including the end face of the electro-optic crystal integrally formed under the crystal portion.
  • the refractive index of a boundary portion between the ridge portion and the base portion is made lower than the refractive index of the electro-optic crystal in the ridge portion, it is possible to prevent diffracted light from leaking even when the length of the electro-optic crystal is lengthened. Therefore, a large phase modulation depth can be obtained.
  • an insulator is applied so as to relatively fix the electro-optic crystal and one pair of electrodes. Therefore, distortion of the electro-optic crystal is suppressed and flat frequency characteristics are obtained.

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EP1526400A1 (en) 2005-04-27
EP1526400A4 (en) 2006-07-05

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